Showing papers on "Reaction rate constant published in 1980"
TL;DR: In this article, the authors derived a differential rate equation for silica-water reactions from 0-300°C based on stoichiometry and activities of the reactants in the reaction SiO2(s) + 2H2O(l) = H4SiO4(aq) ( ∂a H 4 SiO 4 ∂t ) P.T.
1,186 citations
TL;DR: In this paper, the theory of homogeneous redox catalysis in the context of cyclic voltammetry is presented for the EC reaction scheme, i.e., the simplest of the mechanisms in which the initial electron-transfer process is followed by chemical reaction.
309 citations
TL;DR: 2-Nitrobenzyl derivatives have been used for several years as photolabile protecting groups in synthetic organic chemistry and recently, P3-1-(2-nitro phenylethyladenosine 5'-triphosphate "caged ATP" was synthesized and its photolysis was shown to generate ATP in situ.
Abstract: 2-Nitrobenzyl derivatives have been used for several years as photolabile protecting groups in synthetic organic chemistry. Recently, P3-1-(2-nitro phenylethyladenosine 5'-triphosphate "caged ATP" was synthesized and its photolysis was shown to generate ATP in situ. This and related reactions have great potential for structural and kinetic studies of both intact and soluble biological systems and it is thus important to define the kinetic characteristics of the photolytic reaction. Caged ATP (2.5 mM) was photolyzed at 347 nm by a single 30-nsec pulse from a frequency-doubled ruby laser of 25 mJ energy to generate 500 microM ATP. The kinetics of the overall reaction were determined by monitoring the kinetics of ATP-induced dissociation of actomyosin, a reaction of known kinetic characteristics. Release of 500 microM ATP was found to be controlled by a process having a rate constant of 2.2 X 10(9) [H+] sec-1 at 22 degrees C at pH 5.8-9.5, which corresponds to 220 sec-1 at pH 7. This process is believed to be the breakdown of an aci-nitro compound, which was identified on the basis of its spectral properties and the photochromicity of related 2-nitrobenzyl compounds.
232 citations
TL;DR: The surface recombination rate for hydrogen isotopes on a metal has been calculated using a simple model whose parameters may be determined by direct experimental measurements as discussed by the authors, using the experimental values for hydrogen diffusivity, solubility, and sticking coefficient at zero surface coverage.
217 citations
TL;DR: In this article, the catalytic decomposition of formic acid was studied on a (110)-oriented copper single-crystal surface and the rate constant was determined to be 9.4 × 10 13 exp ( −31,870 RT ) sec −1.
196 citations
TL;DR: In this article, it was shown that the isomeric dihydroxycyclohexadienyl radicals formed in aqueous solution by reaction of OH with phenol react with quinones by electron transfer to yield semiquinones and dihydroxbenzenes.
Abstract: Using pulse radiolysis with optical detection and high-pressure liquid chromatography techniques it is shown that the isomeric dihydroxycyclohexadienyl radicals formed in aqueous solution by reaction of OH with phenol react with quinones by electron transfer to yield semiquinones and dihydroxybenzenes. The rate constants for these electron-transfer reactions depend on the redox potential of the quinone and on the isomer structure of the dihydroxycyclohexadienyl radical; i.e., the rate constants are large (10/sup 7/-10/sup 9/M/sup -1/s/sup -1/) in the case of the ortho and para isomers and small (less than or equal to 10/sup 7/M/sup -1/s/sup -1/) in the case of the meta and ipso isomers. The isomeric dihydroxycyclohexadienyl radicals differ also with respect to their rates of H/sup +/-catalyzed dehydration to yield phenoxyl radical. It is thus possible to distinguish kinetically between the different dihydroxycyclohexadienyl radicals. On this basis and from the product analysis data the fractions of OH attack at the ipso, ortho, meta, and para position of phenol are 0.08, 0.48, 0.08, and 0.36, respectively. These data show the strong preference of the electrophilic OH radical for attack at the ring positions activated by the phenolic OH group.
194 citations
TL;DR: The high value of k120 and the space requirements of six to nine charged groups suggest that regions of the enzyme surface area larger than the catalytic sites themselves are effective in trapping cationic ligands.
Abstract: The reaction of the specific fluorescent cationic ligand N-methylacridinium with the active site of 11s ace- tylcholinesterase from electric eel was monitored by temper- ature-jump relaxation kinetics at a variety of ionic strengths. The ionic strength dependence of the bimolecular association rate constant is analyzed with a Brgnsted-Debye-Huckel expression and leads to estimates of the association rate con- stant at zero ionic strength of kI2O = 1.1 X 1Olo M-' s~~ at 25 OC and the net charge number of the enzyme active site of ZE = -6.3. The ionic strength dependence of the second-order hydrolysis rate constant kcat/Kapp for acetylthiocholine under steady-state conditions is also very pronounced and indicates
177 citations
TL;DR: The degradation of S--S bonds in 0.2 M-NaOH at 25 degrees C was studied for a series of proteins and simple aliphatic disulphide compounds, by using cathodic stripping voltammetry, ion-selective-electrode potentiometry, spectrophotometry and ultrafiltration.
Abstract: The degradation of S--S bonds in 02 M-NaOH at 25 degrees C was studied for a series of proteins and simple aliphatic disulphide compounds, by using cathodic stripping voltammetry, ion-selective-electrode potentiometry, spectrophotometry and ultrafiltration The disulphide bonds that dissociated in 02 M-NaOH were usually those that are solvent accessible and that can be reduced by mild chemical reductants Some unexpected differences were found between similar proteins, both in the number of S--S bonds dissociated and in their rates of decomposition Chymotrypsin has one S--S bond attacked, whereas chymotrypsinogen and trypsinogen have two Ribonuclease A has two S--S bonds dissociated, but ribonuclease S and S-protein have three Denaturation in 6 M-guanidine hydrochloride before alkaline digestion caused the loss of an additional S--S bond in ribonuclease A and insulin, and increased the rate of dissociation of the S--S bonds of some other proteins The initial product of S--S bond dissociation in dilute alkali is believed to be a persulphide intermediate formed by a beta-elimination reaction This intermediate is in mobile equilibrium with bisulphide ion, HS-, and decomposes at a mercury electrode or in acid solution to yield a stoichiometric amount of sulphide Rate constants and equilibrium constants were measured for the equilibria between HS- and the intermediates involved in the alkaline dissociation of several proteins Elemental sulphur was not detected in any of the protein digests It is suggested that formation of HS- from a persulphide intermediate involves a hydrolysis reaction to yield a sulphenic acid derivative The small polypeptides glutathione and oxytocin gave only a low yield of persulphide, and their alkaline decomposition must proceed by a mechanism different from that of the proteins
153 citations
TL;DR: In this paper, the production of free radicals by reaction of 2,2,6,6-tetramethyl-4-piperidinol with singlet oxygen was studied by EPR spectroscopy.
Abstract: — The production of free radicals by reaction of 2,2,6,6-tetramethyl-4-piperidinol with singlet oxygen was studied by EPR spectroscopy. The rate constant of the amine was found to be equal to 8 ×105M-1s-1 in ethanol and to 4 × 107M-1s-1 in phosphate buffer (pH 8). Competition experiments were performed with singlet oxygen quenchers such as NaN3, DABCO and the quenching rate constants were found to be consistent with the literature values. The EPR method proved to be a valuable technique to study the reaction of singlet O2 with the sterically hindered amine without any interfering effect.
143 citations
TL;DR: In this article, the photooxidation of a number of organic species, representing several classes of atmospheric pollutants, was studied in the presence of HONO, NO, and NO/sub 2/ at parts per million concentrations.
Abstract: The photooxidation of a number of organic species, representing several classes of atmospheric pollutants, was studied in the presence of HONO, NO, and NO/sub 2/ at parts per million concentrations. The experiments give information on the rate and mechanism of the reaction with organic molecules of HO radicals produced by photolysis of HONO under these conditions. The data allow evaluation of relative rate constants for the reaction of HO with these organic species and the stoichiometry for NO to NO/sub 2/ conversion during the subsequent oxidation steps. The implications of the results for the assessment of the atmospheric reactivity of organic pollutants with respect to photooxidant formation are briefly discussed.
131 citations
TL;DR: The results suggest that the enzyme reaction involves the sequential formation of at least three forms of acid-stable EP, viz. in the order of formation, EPD+, EPD-M-, and EPD-,M+.
TL;DR: In this article, the rate equation for Fe(III)-oxyhydroxides was derived, where the rate is the product of the equilibrium constant for the adsorption of Fe2+ (K) and the specific rate for the oxidation of adsorbed Fe 2+ (ks), and the values for K and ks have been determined as functions of pH and [O2] for each Fe (III) species.
TL;DR: In this article, the reaction path Hamiltonian model was applied to the unimolecular isomerization HNC→HCN and the energy and force constant matrix along it was calculated at the SCF level with a large basis set (double zeta plus polarization) and the microcanonical rate constant k(E) was computed in the energy range near and below the classical threshold.
Abstract: The reaction path Hamiltonian model recently formulated by Miller, Handy, and Adams [J. Chem. Phys. 72, 99 (1980)] is applied to the unimolecular isomerization HNC→HCN. The reaction path, and the energy and force constant matrix along it, are calculated at the SCF level with a large basis set (’’double zeta plus polarization’’), and the microcanonical rate constant k(E) is computed in the energy range near and below the classical threshold of the reaction. It is seen, for example, that rates as fast as 105 sec−1 can occur at energies ∼8 kcal/mole below the classical threshold.
TL;DR: In this paper, an extensive reaction set has been used to probe the sensitivity of the H2-O2 system in a wide pressure-temperature range, and the results indicate the present model to be useful over a wide range of pressure and temperature.
Abstract: Kinetic modeling calculations on the H2–O2 system have been carried out with an extensive reaction set to probe the vicinity of the three explosion limits. Sensitivity analysis is used throughout this investigation to study system behavior, in particular, to elucidate mechanistic details. The concentrations and sensitivity profiles are discussed in light of the appropriate experimental results and existing theories of hydrogen combustion. The results indicate the present model to be useful over a wide pressure–temperature range. The reaction set is also used to probe the sensitivities for an experimental study designed to measure the rate constant of an important elementary reaction, H+O2+M→HO2+M, involved in this system. The versatility of the reaction set is also demonstrated by a study of a related chemical reaction, the decomposition of hydrogen peroxide. Finally, prospects for utilizing the methods and results of this study to examine other complex kinetic schemes are discussed.
TL;DR: The ionic strength dependence of the slow kinetics of oxidation of cytochromes c and c2 confirmed the electrostatic nature of the cytochrome-reaction center interaction, and the pH dependence indicated the titration of a group or groups, important to this interaction, at pH 9.5.
Abstract: The oxidation of cytochrome c2 by photosynthetic reaction center isolated from Rhodopseudomonas sphaeroides and incorporated into unilamellar phosphatidylcholine vesicles was found to be kinetically similar to that observed earlier for reaction centers in low detergent solution [Overfield, R.E., Wraight, C.A., & DeVault, D. (1979) FEBS Lett. 105, 137-142]. At low ionic strength the kinetics were biphasic. The fast phase indicated the formation of a cytochrome-reaction center complex with an apparent binding constant, KB, of about 10(5) M-1. However, KB decreased dramatically with increasing salt concentration, and no fast oxidation was detectable in 0.1 M NaCl. The slow cytochrome oxidation was first order in both cytochrome and reaction centers and, thus, second order overall. Deviations from theoretical second-order behavior were observed when the rate of the first-order back reaction of the primary photoproducts was significant compared to the cytochrome oxidation. This can cause serious overestimation of the second-order rate constant. The slow oxidation of cytochrome c2 by reaction centers in phosphatidylcholine vesicles exhibited a 40% lower encounter frequency than with the solubilized reaction center. This was attributed to the much lower diffusion coefficient of the reaction center in the vesicle membrane than in solution. No effects of diminished dimensionality were detected with neutral vesicles. An activation energy of 8.0 +/- 0.4 kcal x mol-1 was determined for the slow phase of cytochrome c2 oxidation by reaction centers in solution and in vesicles of several different phosphatidylcholines, including dimyristoylphosphatidylcholine above and below its phase transition temperature. Thus, the physical state of the lipid did not appear to affect any rate-limiting steps leading to cytochrome oxidation. The ionic strength dependence of the slow kinetics of oxidation of cytochromes c and c2 confirmed the electrostatic nature of the cytochrome-reaction center interaction, and the pH dependence indicated the titration of a group or groups, important to this interaction, at pH 9.5.
TL;DR: The dependence of the rate constant on solvent polarity is an important mechanistic criterion which should always be used in conjunction with other diagnostics tool as discussed by the authors. But the violation of stereo-specificity becomes greater with increasing solvent specificity.
Abstract: The dependence of the rate constant on solvent polarity is an important mechanistic criterion which should always be used in conjunction with other diagnostics tool. The slow step of 2 + 2 cycloadditions of tetracyanoethylene (TCNE) with enol ethers [1], thioenol ethers [2] and trans-fixed 1,3-dienes [3] is the formation of a zwitterionic intermediate. The log k2 values are linear functions of the Dimroth-Reichardt parameter ET. The rate accelerations of TCNE cycloadditions in going from cyclohexane to acetonitrile amount to 29,000 for anethole, 10,800 for 1-ethoxyisobutene, 2,600 for butyl vinyl ether, 17,000 for ethyl 1-propenyl sulfide and 54,000 for verbenene. The violation of stereo-specificity becomes greater with increasing solvent polarity.
In contrast, the additions of TCNE to anthracene [4] and related Diels-Alder reactions show only a minute solvent dependence. The same is true for 1,3-dipolar cycloadditions of diazoalkanes [5], phenyl azide [6], C-phenyl-n-methylnitrone and azomethine imines to various dipolarophiles. These concerted cycloadditions are characterized by early transition states, i.e., there is hardly any change of solvation energy during the activation process.
A dichotomy of reaction paths was found for diazocarbonyl compounds and enamines. Whereas dialkylaminocyclohexenes produce cycloadducts, the corresponding enamines containing the cyclopentene ring undergo azo coupling furnishes enamino-hydrazones via a zwitterionic intermediate. The contrasting dependence of rate on solvent polarity supports different mechanism [7]. Dimethylketene combines with N-isobutenylprrolidine to give a 3-pyrrolidinocyclobutanone as 1:1 adduct and a δ-methylene-δ-lactone as 2:1 adduct. The dependence of the product ratio on the concentration of dimethylketene allows to disent angle a concerted pathway and a reaction via a zwitterionic intermediate which can be intercepted by a second molecule of dimethylketene (see formula Scheme) [8]. The different solvent dependencies of the two reaction branches confirm the mechanistic divergence.
TL;DR: In this paper, the first-order rate constants for reactions of the methoxy and ethoxy species were determined to be kCH3O = (4 × 1012) exp(−105 kJ/mole/RT)s−1kCH3CH2O =(8 × 1013) exp((−111 kJ /mole)/RT) s−1, respectively.
TL;DR: In this article, the rate constants for the reaction of four different butyl radicals with molecular oxygen have been measured at room temperature, and the radicals were generated by flash photolysis and their time decay was followed with a photoionization mass spectrometer.
Abstract: Rate constants for the reaction of four different butyl radicals with molecular oxygen have been measured at room temperature The radicals were generated by flash photolysis and their time decay was followed with a photoionization mass spectrometer The radical concentrations were kept low to avoid complications from radical–radical reactions Radical lifetimes were long, up to 50 msec, thus assuring that thermalized radicals were being studied The rate constants, in units of 10−11 cm3 molecule−1 sec−1, are: n‐butyl (075±014); s‐butyl (166±022); t‐butyl (234±039); 3‐hydroxy s‐butyl (28±18) No pressure dependence of the rate constants was observed over the range 1 to 4 Torr In the absence of O2, the butyl radicals decay mainly by loss on the quartz surface of the reaction cell, with sticking coefficients in the range of 10−2 to 10−3 The Adiabatic Channel Model can predict the approximate absolute values of these rate constants using reasonable molecular parameters, but it fails to reproduce th
TL;DR: In this paper, general expressions for the kinetics of a 1 e, 1H + surface electrochemical reaction are established under the following conditions: all the participants are strongly adsorbed; in every case a Langmuir isotherm is obeyed; the protonation reactions are at equilibrium.
TL;DR: In this paper, the authors examined the reactions of metastable O+* ions with N2 and O2 in a selected ion flow-drift tube over the relative kinetic energy range from thermal to 0.2 eV.
Abstract: The reactions of metastable O+* ions with N2 and O2 have been examined in a selected‐ion flow‐drift tube over the relative kinetic energy range from thermal to 0.2 eV. The rate constants are found to be (8.5±3.4)×10−10 and (8.1±3.2)×10−10 cm3 s−1, respectively, at these energies. These values are essentially equal to the gas‐kinetic limiting values. The principal reaction mechanism in both cases is charge transfer. Based on molecular beam studies of the ion‐source reaction thought to produce the present O+* ions, it appears likely that they are largely O+(2D°) ions. The present results for these reactions are in good agreement with the results of most of the earlier studies, which were limited to either thermal or elevated collision energies. However, the present data for O+*+N2 appear to conflict with the much smaller rate constant predicted for the reaction of O+(2D°)+N2 from analyses of satellite data. As a necessary part of this study, the mobilities of the O+* ions in helium were determined and are 7...
TL;DR: In this article, a linear relationship between the variables log A and E was established, which is known as a compensation effect, expressed by the following equation, where w is the weight or mass of pyrolyzing sample at any time t; A is the frequency factor; E is the activation energy; Q is the reaction order, and k is the rate constant.
TL;DR: In this paper, the first order rate constants for pyrolysis are 3·7 × 10−4 sec−1 and 4·0 × 10 −4sec−1 for atactic and isotactic polypropylene at 388°C; the corresponding overall activation energies are 56 ± 6 and 51 ± 5 kcal mole−1.
TL;DR: Despite the favourable thermodynamic conditions for the production of all beta-lactam antibiotics, low reaction rate is the basic hindrance for enzymatic synthesis of penicillins and cephalosporins having a free amino group in the acyl moiety.
TL;DR: In this paper, the acid catalyzed hydrolysis of the ester group is considered and the fractional rate of increase in acidity, k, is a pseudo first order rate constant because the esters and water contents change only slightly with degradation.
Abstract: : The hydrolytic degradation of polyester urethane elastomers is due to the acid catalyzed hydrolysis of the ester group. Acid content (A) and the reciprocal of the number average molecular weight increase by equivalent amounts indicating that each molecular chain scission produces one acid group. The fractional rate of increase in acidity, k, is a pseudo first order rate constant because the ester and water contents change only slightly with degradation. Values of k at 85 C and 100% relative humidity are in the 15-20% per day for polyurethanes, based on polycaprolactone and polytetramethylene adipate diols. At 100% relative humidity k obeys the Arrhenius equation between 35 and 85 C with activation energies in the range 68-83 kJ/mol (16-20 kcal/mol). Values of k decrease with relative humidity, becoming zero in dry air.
TL;DR: In this paper, the magnetic field effect on the decay rate of the benzophenone ketyl radical in micellar solution was studied via nanosecond laser photolysis, and the rate constant of the slow component was found to decrease from 1.6 × 10 6 s − 1 at zero field to 9 × 10 5 s −1 at 700 G.
TL;DR: In this paper, the electrochemical behavior of cis and trans-azobenzenes in buffered protic solvents has been studied; the reduction always occurs in a one-step, two-electron recction.
TL;DR: In this paper, the reduction of tert-nitrobutane in acetonitrile has been studied by cyclic voltammetry by the fit of data to theoretical voltammograms obtained from digital simulations.
TL;DR: In this paper, the temperature dependence of the F+HCl, HBr, F+DBr, and F+HI absolute rate constants in the range from 195 to 373°K was studied.
Abstract: A laser photolysis/infrared fluorescence technique has been used to study the temperature dependence of the F+HCl, F+HBr, F+DBr, and F+HI absolute rate constants in the range from 195 to 373 °K. Multiphoton dissociation of SF6 with a TEA CO2 laser was used to provide a transient concentration of fluorine atoms. Their subsequent reaction with hydrogen donors was monitored by observing the appearance rate of HF(v?1) through its infrared fluorescence. The observed rate constants show a marked nonlinear Arrhenius behavior which is consistent with a model in which reaction takes place both vai direct attack by the fluorine atom at the hydrogen end of HX and via formation of a collision complex FXH which rearranges to FHX and then dissociates to HF and X products.